In the manufacture of compositions for use as friction facings for brake shoes and clutch plates, including automatic transmission torque converter clutch plate applications, the friction composition is characterized generally as a paper-like article which is bonded to the metal shoe or clutch plate member. Generally, paper-like compositions contain several non-functional additives to aid in the manufacture thereof. Such additives are used to improve the retention of fine particles during the paper forming process, provide uniformity of the paper composition and increase the wet and dry strength and stiffness of the resulting paper sheet.
While so-called non-functional additives result in improved processing or manufacture of paper, they generally diminish the finished paper""s physical properties. For example, almost all organic, non-functional processing additives used in paper manufacture tend to decompose at temperatures required for bonding the paper to clutch plates or similar structural members. Moreover, binding resin wetting of the fibers and particles of the paper composition is also severely compromised since the non-functional processing additives coat the outer surface of the paper fibers and particles and, during bonding, the resin bonds to the additive coatings and quickly cross links with such coatings. Subsequently, the non-functional processing additive layers begin to decompose leaving a gap between the now cured resin and the surface of the paper particles or fibers. The resulting paper composition thus has a porous microstructure which causes inferior tensile and shear strength of the composition, particularly for applications as friction brake or clutch facings, for example.
Accordingly, there has been a need to develop paper compositions and processes, particularly for applications such as friction surfaces, including transmission clutch plate applications, by providing a composition having superior wet and dry strength. It is to these ends that the present invention has been developed.
The present invention provides an improved paper composition, particularly adapted for friction surface applications including, for example, facings for automatic transmission torque converter clutch plates, brake shoes or disks and similar friction member applications. The present invention also provides an improved process for making a paper composition having superior wet and dry strength properties.
In accordance with one aspect of the present invention, a paper composition is provided which includes a powdered binder that melts during the paper drying process and replaces typical latex based adhesives. The binder is also used to bond the resulting paper sheet to support structure in applications of the paper sheet as clutch plate friction facings, for example. The binder is typically present in the paper composition in weight quantities substantially greater than prior art adhesive additives. In forming the paper, the wet strength is further increased using conventional paper making methods of vacuum extraction and warm rolling, for example. These processes serve to densify the paper sheet and allow the melted binder to flow. Moreover, fine particle retention is improved by precompounding at least some of the components of the paper composition with the binder wherein the precompounded composition may then be ground to a diameter comparable to the mesh size of the screen used in the paper forming apparatus. In this way, the fines of the composition may be efficiently trapped during formation of the paper without the need for further additives, including flocculents.
The present invention still further provides an improved process for making paper material particularly useful for power transmission equipment friction surfaces, for example, which process eliminates the requirement for liquid resin saturation of the material, results in relatively low outgassing during formation of the material and results in friction material with improved strength, more uniform density, improved resin bonding to the fibers and particles of the paper composition and allows heating to partial or fully cure the resin and the paper composition to a final density while also bonding the material to clutch plates, disks or similar structures used in mechanical power transmission and brake equipment.
Those skilled in the art will further appreciate the advantages and superior features of the invention, together with other important aspects thereof upon reading the detailed description which follows.
The manufacture of papers used for friction materials normally requires compositions which include large quantities of thickeners, such as guar, alum, starch and certain flocculents. Dispersed latex adhesives are also often used and are particularly detrimental to applications of papers for friction materials, such as brake linings and clutch plates, for example.
Heretofore papers used in forming friction faces on clutch plates and the like have been saturated with liquid resin prior to bonding the paper articles to the supporting structure, such as a clutch plate or disk. Moreover, during the curing or bonding phase at elevated temperatures, so-called nonfunctional or processing additives used in forming the paper bond to the resin coating and then often decompose, leaving a minute gap or space between the cured resin and the particles and fibers which make up the paper composition. This action results in a friction material with inferior tensile and shear strength and, consequently, a short working life.
Moreover, conventional paper manufacturing processes have been used in forming papers for friction surfaces, such as brake linings and clutch plates. However, one important aspect of the invention comprises forming a relatively thick slurry in the paper making process. Slurries having 25% to 40% solids and particle sizes as large as 250 to 300 microns have been formed with negligible separation, even when the slurry is allowed to rest without agitation for substantial periods of time. The relatively thick slurries are also conducive to paper making processes of the so-called pulp mold type, which processes may be advantageously used in the manufacture of articles such as ring shaped clutch plates or brake discs while minimizing waste of the paper material.
In accordance with the invention, paper compositions have been developed which have eliminated many nonfunctional processing additives in the compositions. Still further, the improved paper compositions have been precompounded with binding resins, particularly cyanated phenolic resins, for example. The resin is precompounded with the other materials making up the paper composition and in the precompounding step, the resin and other materials making up the composition are ground to a diameter comparable to the mesh size of the screen of the paper making apparatus. Accordingly, the paper particles may be efficiently trapped by mechanical filtration and without the need for modifiers and flocculents in the paper composition.
The ingredients of the following examples were precompounded to particle sizes of about 60 mesh (250 micrometers). Particle sizes substantially larger than 60 mesh are not efficiently dispersed and particles smaller than about 200 mesh are not efficiently trapped unless a 150 mesh screen and/or a thick paper sheet ( greater than .55 grams per square inch) is formed. Accordingly, the precompounded particle size distribution is xe2x88x9260/+200 mesh, preferably centered at 100 mesh. Smaller particles are efficiently trapped as soon as a mat of the bigger particles is deposited on the paper making screen.
In the examples below precompounding was carried out with a cyanated phenolic resin commercially available as composition CT 90 from Lonza, Inc., Annadale, N.J. This resin is free flowing at room temperature and is an excellent choice for a free resin addition to the composition or a very lightly loaded precompound without the need for a partitioning agent. Precompounding may be carried out in a cryo mill, although not necessary, and may be carried out with a low resin content wherein the remainder of the resin may be introduced as another powdered phase during the actual paper making process. Several advantages are realized. The melt flow viscosity of the precompounded material is not substantial. The fines tend to stay as isolated islands while the resin squeezes out to coat the nearest particles. Resin content can be reduced by about 20% without adversely affecting the precompounding process.
The following examples were formed by hand and using a Rotoformer type paper manufacturing apparatus with a premixing tank, including a motor driven, high pitch, relatively short blade propeller type mixer, having a geometry not unlike an outboard motor type propeller.